NEW YORK (CNN/Money) -
A mistake in the minting process for some quarters issued last year is putting coin collectors in a frenzy. Speculators are bidding up prices for the recently discovered pieces from their 25-cent face value to nearly $1,500.

The coin in question is the Wisconsin quarter, part of the U.S. Mint's ongoing "50 State Quarters" commemorative series. Minted in 2004, it bears the image of a cow, a wheel of cheese, and a half-husked ear of corn.

Last month, the trade publication Coin World reported, numismatists in Arizona noticed something peculiar about the cheesehead coins: they weren't all uniform. At least two versions had been minted with noticeable -- if ever-so-slight -- variations from the official engraving.

The variation involves the placement of the leaves surrounding the coin's ear of corn. A few quarters that rolled out of the U.S. Mint's Denver production facility appear to have too much husk on them.

"Hobbyists have identified two 'varieties,' generally described as 'Extra Leaf Up' or "Extra Leaf Down.'" Coin World noted. "The unusual coins have raised lines along the left side of the ear of corn on the reverse that some say resemble 'leaves.'"

Those are the kinds of rare distinctions that collectors often love.

In January, a few coin dealers in Tucson reported the discovery of the errant coins, prompting interest within the numismatic community. Since then, publicity surrounding the pieces has drawn attention from a wider circle.

By February 9, Coin World reported, Arizona coin dealers were offering as much as $1,499 to obtain a perfect specimen of the imperfect quarter.

The U.S. Mint, the arm of the Treasury Department responsible for minting coins, does not yet know how the variant coins made it into circulation. An internal investigation is underway to determine whether the mistakes were inadvertent or intentional.